We generate indistinguishable photon pairs in an all-waveguide geometry. We use the source to distribute polarization entangled photons spanning 50 km telecom fiber violating Bell-CHSH inequality with S parameter being 2.53 ± 0.003.
The scalability of quantum communication networks requires compact, fiber-integrated, easy-to-deploy, and efficient wavelength-division-multiplexed (WDM) sources for multi-user secure key distribution. Here, we demonstrate such a multi-channel source of polarization-entangled photon pairs in the low-loss telecom C-band based on type-0 spontaneous parametric downconversion in a fiber-coupled Zn-indiffused MgO-doped periodically poled lithium-niobate (MgO:PPLN) ridge waveguide in the Sagnac configuration. The source can be easily tuned to generate the |Φ+⟩ or |Φ−⟩ Bell state in 14 channel pairs of the International Telecommunication Union dense WDM (100-GHz spacing) grid around 1550-nm with a raw fidelity of ≳89%, with a maximum value of ≳94%. The raw concurrence is ≳0.8 for both Bell states in all channel pairs, and the observed S-parameter (>2.56 ± 0.04 in all 14-channel pairs) shows a strong violation of CHSH-Bell’s inequality. The source’s suitability for long-distance entanglement transmission is also demonstrated by the successful transfer of entangled photons up to 100 km while maintaining fidelity >85% and quantum bit error rate <9%. The effect of polarization mode dispersion on entanglement distribution among remote users is also studied in detail. All these performance metrics are measured using conventional room-temperature semiconductor-based single-photon avalanche detectors, and these are the best reported with these detectors. Our highly flexible source can support up to ∼40 user pairs to communicate simultaneously, and it can be easily deployed into the current metro-area fiber-optic telecom infrastructure to form a complete WDM-based quantum communication network.
We experimentally demonstrate multi-user distribution of polarization entanglement using commercial telecom wavelength division demultiplexers. The entangled photon pairs are generated from a broadband source based on spontaneous parametric down conversion in a periodically poled lithium niobate crystal using a double path setup employing a Michelson interferometer and active phase stabilisation. We test and compare demultiplexers based on various technologies and analyze the effect of their characteristics, such as losses and polarization dependence, on the quality of the distributed entanglement for three channel pairs of each demultiplexer. In all cases, we obtain a Bell inequality violation, whose value depends on the demultiplexer features. This demonstrates that entanglement can be distributed to at least three user pairs of a network from a single source. Additionally, we verify for the best demultiplexer that the violation is maintained when the pairs are distributed over a total channel attenuation corresponding to 20 km of optical fiber. These techniques are therefore suitable for resource-efficient practical implementations of entanglement-based quantum key distribution and other quantum communication network applications.